Summary: Ribosomes are responsible for the rapid and accurate production of all the thousands of different proteins in cells in all forms of life on earth. The ability of these molecuar machines to do this depends upon a complex structure that is both stable and flexible, allowing it to interact with ligands in a dynamic fashion. The mature ribosome has binding sites for multiple ligands, including initiation factors, elongation factors, termination factors, tRNAs and mRNA, and it must ratchet between multiple stable conformations, moving tRNAs in and out at enormous rates. The accurate assembly of a fully functional eukaryotic ribosome involves over 200 accessory assembly factors, whose function, in many cases, is still unknown. However much recent excitement has focused on the role of some of these proteins in quality control checkpoints that occur late in the biogenesis process in the cytoplasm. We have recently proposed that the 60S subunit undergoes a cytoplasmic test drive in which newly assembled ribosomes are tested for function by biogenesis factors that structurally mimic translation factors. This proposal seeks to understand the mechanism of 60S maturation, focusing on the last steps of this pathway, the release of Tif6 and Nmd3, which, we believe, comprise the primary quality control checks on the large subunit. This proposal is directed at understanding quality control mechanisms that assess the functional and structural integrity of the large ribosomal subunit. This proposal builds on our recent results from the previous funding period suggesting that the newly assembled subunit undergoes a test drive that uses molecular mimics of translation factors prior to bona fide translation. In collaboration with Dr Jan Cools (VIB, Belgium) we have also recently shown that a defect in this quality control check is associated with ~8% of pediatric acute T-cell leukemia samples. Defects in this step are also associated with Shwachman-Diamond syndrome, an inherited disease that manifests in bone marrow failure, pancreatic dysfunction and predisposition to cancers among other conditions. These results highlight the importance of understanding the mechanism of this quality control step for its potential target for new drug therapies.